Sugarcane growth and yield is badly affected by soil sodicity in considerable areas in NSW and QLD. Adding gypsum, which is a relatively soluble source of calcium, can often reduce soil sodicity. The optimum amount to add depends on several soil properties which can be determined from soil testing.
Soil sodicity has been conservatively estimated to cost the Australian sugar industry 500,000 tonne of cane per annum (Ham et al., 1997). Approximately 10-24% of the cane-growing soils
outside of the wet tropics are sodic, and the proportion is higher in areas into which the industry is expanding. Trials, summarised by Ham et al. (1997) have shown have shown gypsum
applications of up to 10 t/ha to be economic on sodic soils.
Gypsum quality and pure gypsum application rate:
The rate of pure gypsum (CaSO4.2H2O) applied is calculated based on the S content of the applied gypsum. The pure gypsum application rate is used in the subsequent calculations.
Sulphur (S) content is used as a measure of gypsum purity rather than calcium (Ca). The rate of gypsum applied should be expressed as dry weight, allowing for water content. Some available NSW and QLD mined gypsum products can have S contents as low as 12%, whereas REGYP gypsum products are all over 17%. Pure gypsum has a S content of 18.6%.
Particle size also influences the rate at which gypsum dissolves into solution. REGYP’s recycled gypsum can be up to 4 times more soluble than the REGYP natural mined due to the increased surface area in the recycled gypsum particles.
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